In beamformed or steerable antenna systems, such as may be used in base stations for cellular telephone networks, an antenna may be comprised of an array of identical antenna elements.
In one such design, known as a cavity backed, slot fed dual polarized patched element, the antenna element comprises, in order from the back of the radiating element to the front, a cavity structure, a dual feed network, a pair of slots and a patch radiator.
The cavity ensures that all of the radiated energy emerges from the front of the antenna element.
The dual feed network is largely to provide the necessary fields to drive the slot elements by exciting the appropriate field structure on the patch radiator.
The slots in turn excite the necessary fields for the dual polarized patch elements.
The patch radiator is the active or radiating part of the antenna element. The size and configuration of the patch radiator has a significant impact on the operating characteristics of the antenna element.
However, in beamformed antenna arrays, the spacing between the centres of adjacent rows and/or columns imposes a performance constraint. For example, those skilled in the relevant art will understand that exceeding array spacing threshold maxima may introduce grating lobes in the radiated signal, which is generally undesirable. As an exemplary rule of thumb, array elements may be restricted to no more than 0.5 wavelength spacing in the azimuthal plane and 0.8 wavelength spacing in the elevation plane. The greater wavelength spacing in the elevation plane is generally considered acceptable because typically the narrow beamwidth and low skew angle of the beam provides assistance so that the undesirable grating lobes cannot form.
Leaving aside the performance implications, it is generally desirable to optimize the array element spacing so as to produce an antenna array with a small physical footprint consistent with the required radiation patterns.
Therefore, care should be taken to design a patch element that provides satisfactory performance while satisfying the various design criteria of the radiating element. For example, it is generally accepted that for dual polarization elements, the two polarizations are set at +/−45°. This generally implies that a square patch radiator be oriented along a diagonal relative to the array.
As well, the antenna element should be designed to provide a suitable frequency bandwidth to accommodate the application for which it is intended.
It is generally understood, at least in a colloquial or empirical sense, if not strictly proven by electro-magnetic field calculations, that for patches that are defined by polygonal shapes that have no interior angles of less than 180°, the operating frequency is determined by the perimeter of the patch element. Thus, in order to minimize physical size of the patch, it is generally preferable to maximize the area enclosed relative to the enclosing perimeter. As such, typical patch shapes that have been successfully employed include square or rectangular patches, such as is shown in FIG. 1A. Other patch shapes include circular patches, such as is shown in FIG. 1B.
It is also generally understood, in the empirical sense at least, that the EM characteristics of such patches impose, as a design objective, that the patch perimeter may be on the order of 1.5 wavelengths in length.
On the other hand, it has been found that removing some patch material from the interior of the patch shape has an ameliorating effect on its EM characteristics such that, as a rule of thumb, the patch perimeter may be reduced to be on the order of 1.0 wavelengths in length. Clearly, this has salutary benefits for the antenna designer, who is constrained to minimize, so far as possible, the inter-element spacing of the antenna array.
This latter observation has resulted in a second generation of patch radiators, wherein the interior annular region of the patch element adopts the shape of the exterior perimeter so that the amount of material between the inner annular region and the exterior perimeter remains constant. Thus, for example, an exemplary conventional annular patch radiator might be a square with a corresponding square interior annular region of removed conductive material, such as is shown in FIG. 1C. For this class of annular patches the centre frequency is known to be inversely proportional to the inner and outer perimeters respectively. Another example might be a patch of circular shape, with an interior circular annular region of removed material, such as is shown in FIG. 1D.
The similarity of shape between the interior annular region and the exterior perimeter ensures that there is a relatively constant amount of material in the radiator as one proceeds along the exterior of its perimeter.
However, it has been found that the threshold upper frequency limit tends to increase in proportion to the ratio of the area of removed material defined by the interior annular region to the perimeter of such interior annular region. Accordingly, there is a need for an improved patch radiator configuration which maximizes upper frequency limit and simultaneously minimizes the lower frequency limit. In this regard, the present invention substantially fulfills this need.